Unable to mount read-write in Big Sur 11.2.2 in Single User Mode

I'm on Big Sur 11.2.2, Single User Mode. When I use /sbin/mount -uw /, it says mount_apfs: volume could not be mounted: Permission denied and failed with 66. I have read [this post](https://apple.stackexchange.com/questions/388469/unable-to-mount-read-write-in-os-catalina-10-15-4-in-single-user-mode) . But mount -uw /System/Volumes/Data gives mount: unknown special file or file system /System/Volumes/Data/. Also, csrutil enable --without fs in recovery mode changes nothing. Errors still occurs. (Previous tries are based on SIP entirely disabled.) The commands mount -P 1; /usr/libexec/init_data_protection; mount -P 2 in the prompt during startup works. However, I don't know what it's mounting. PS: current SIP status:

Configuration:
	Apple Internal: disabled
	Kext Signing: enabled
	Filesystem Protections: disabled
	Debugging Restrictions: enabled
	DTrace Restrictions: enabled
	NVRAM Protections: enabled
	BaseSystem Verification: enabled

This is an unsupported configuration, likely to break in the future and leave your machine in an unknown state.